Flexographic and gravure printing were the traditional and long-time processes for printing on flexible substrates for packaging, labels, bags and the like. In the recent years, volumes of individual printing jobs went down from over multiple 100.000 of copies to several 1000 copies. This tendency is growing and is driven by the need to prevent dead capital in printed stock and by the need for quick reaction on events in the form of action packs and the higher degree of diversity of packed products. These new demands from the market made clear that flexographic and gravure printing have several shortcomings to match with these new demands:
1) Print form costs per square meter printed substrate are too high for medium and short run jobs. The image carriers or clichés used in flexographic and gravure printing are expensive, which increase the square meter price of the printed substrate up to unacceptable levels.
2) Production time of the print form is too long with respect to actual demands. The production of the printing forms used in flexographic and gravure printing is time consuming, which has a negative impact on the flexibility of the printing process (job shifts are time consuming) and on time-to-market, and can create long down time of the press.
3) Environmental issues due to solvents and energy consumption. The inks used by flexographic and gravure printing are mainly solvent based, being VOC's (Volatile Organic Compounds) like toluene or water. There is a strong environmental impact from the VOC's, and blowing off is not longer allowed. Regeneration of the VOC's is possible but only at high additional costs. In case of water-based inks, taking the water from the ink goes only with high energy consumption, therefore also very high costs.
The three previously mentioned main disadvantages are completely solved with the introduction of web-offset printing in combination with radiation curing ink technology and making use of easy exchangeable printing cylinders allowing variable repeat lengths. The advantages are as follows:
1) Low cost image carrier (offset plate). The cost of an image carrier (offset plate) in offset is much lower in comparison with a flexographic cliché or an engraved cylinder for gravure printing.
2) Very fast production of image carrier (offset plate). Short pre-press times for production of offset printing plates is a second important advantage over the traditional printing processes like flexographic and gravure.
3) No solvents involved. UV (Ultraviolet) and EB (Electron Beam) curable inks are used for offset printing which, when cured, are 100% converted from liquid into solid state without any use, or loss, of solvents.
For example patent EP1101611 discloses a web-offset printing machine for printing on flexible substrates making use of easy exchangeable lightweight printing cylinders (sleeves) which are individually servo driven for variable repeat lengths, and making use of UV or EB curable inks. Web-offset printing machines built according to the previous patent or with similar characteristics, are designed as in-line printing machines, meaning all printing stations lay in the same horizontal level. In between offset printing stations the web printable substrate is not supported in any way. For a controlled transport of the web from one offset printing station to the next, and for accurate color-to-color print register, the in-line printing machine configuration requires a certain minimum web-tension. Under these circumstances thinner substrates with high elasticity, like for example PE (Polyethylene) and CPP (Cast Polypropylene), have the tendency to stretch more then acceptable, with a short image and poor color-to-color register as a result. For these reasons the in-line printing machine configuration is only suitable for a limited amount of substrates, with a low elasticity, like for example PET (Polyethylene Terephthalate) and OPP (Oriented Polypropylene) amongst others.
In flexographic printing it is common use to arrange the flexographic printing stations around a central impression cylinder. The advantage of a central impression cylinder is that a high elastic substrate, once it is positioned onto the surface of the central impression cylinder, is more or less fixed and therefore the web printable substrate doesn't stretch during printing, resulting in a correct repeat length and accurate color-to-color register.
An obvious solution to combine offset printing with a central impression cylinder would be to position the offset printing cylinders and the offset printing station in a radial way around the central impression cylinder. In this situation the axes of plate and blanket cylinders of each printing station are exactly or almost exactly in the same plane than the axis of the central impression cylinder, which is an advantage for quality printing in regard to gap-bounce issues. But with the provision of at least 6 to 8 printing stations on average for packaging and label printing and the necessary scale of printing cylinders and printing stations required by market (speed) and quality (stability) demands, this would have a negatively impact on the total height of the press.
Patent ES-A-2319952 discloses a printing machine comprising a plurality of offset printing stations arranged around a central impression cylinder. Each printing station comprises a blanket cylinder supported on a first support, a plate cylinder supported on a second support and an offset inking unit supported on a third support, wherein said first, second and third supports are linearly movable in horizontal directions between respective withdraw positions and respective multiple working positions able to accommodate and work with pairs of blanket and plate cylinders of different diameters to allow a variable repeat length. Such a horizontal linear displacement of the blanket and plate cylinders and offset inking units is advantageous to keep the total height of the press within desirable limits. The horizontal movement of the offset printing cylinders and offset printing stations also allows for identical construction of all the printing stations, reducing the total amount of different parts, which is a clear advantage regarding costs and logistics.
However, a drawback with cited patent ES-A-2319952 is that the axes of the plate cylinder and blanket cylinder of each printing station are comprised in a horizontal plane while the axes of the blanket cylinder and central impression cylinder are comprised on an inclined plane which forms an angle with the horizontal plane that is greater the higher or lower is the position of the printing station with respect to the axis of the central impression cylinder. The greater is said angle the more sensitive is a minimal movement of the cylinders caused by so called gap-bounce consisting of a short and sudden drop and rise of the pressure between the printing cylinders when a gap existing in a blanket supported on the blanket cylinder passes through the nip with the plate cylinder and through the nip with the central impression cylinder. This movement can cause stripes in the print.
U.S. Pat. No. 718,172 discloses a variable format offset printing machine having a plurality of left and right stacked opposite printing stations arranged to print on both sides of a web printable substrate passing through the nip of pairs of blanket cylinders of the opposite pairs of printing stations. Therefore, there is neither a central impression cylinder nor individual impression cylinders. The opposite blanket cylinders of the left and right printing stations are mounted on a fixed central frame and the plate cylinders together with the corresponding offset inking units of the left and right printing stations are mounted on respective left and right frames horizontally movable between withdrawn and working positions with respect to the fixed central frame. Thus, when the left and right frames are in their withdrawn positions a clear access is provided to the blanket and plate cylinders.
A drawback with the cited patent U.S. Pat. No. 718,172 printing machine is that there is no access to the rollers and other components of the offset inking units neither with the left and right frames in their withdrawn or working positions. If both blanket and plate cylinders of each printing station were mounted on the fixed central frame, then no good accessibility is provided for changing the format, i.e. replacing the pairs of blanket and plate cylinders by others having a different diameter. Additionally, the printing machine of cited U.S. Pat. No. 718,172 has the drawbacks related with the lack of central impression cylinder when working on a high elastic web substrate, as discussed above.